K. B. Urquhart

Large surface anisotropies in ultrathin films of bcc and fcc Fe(001) (invited)

Large uniaxial anisotropies associated with interfaces are observed for ultrathin films (3‐28 ML) of bcc Fe(001) grown epitaxially on Ag(001) single‐crystal substrates and for epitaxial sandwiches of fcc Fe(001) grown with three layers of Fe using Cu as substrate and coverlayers. The uniaxial anisotropy is well described by a pseudosurface anisotropy term as theoretically predicted, yet that theory also predicts large in‐plane anisotropies that are not observed. Adequate treatment of spin‐orbit coupling in magnetic theories remains a challenge. Comparisons of ultrathin films of bcc Fe(001) on Ag(001) with different coverlayers of Ag or Au show subtle differences in magnetic behavior as studied by ferromagnetic resonance (FMR) and Brillouin light scattering (BLS). The FMR measurements were carried out at 9.6, 36.6, and 73 GHz microwave frequencies. The BLS measurements were performed using a six‐pass Fabry–Perot interferometer. The power of the techniques of molecular‐beam epitaxy (MBE) for producing well‐...

Using ferromagnetic resonance to measure the magnetic moments of ultrathin films

Abstract A technique that uses ferromagnetic resonance (FMR) to measure the saturation magnetization of ultrathin ferromagnetic films is described. It has been used to measure the layer averaged magnetic moments of ultrathin Fe films located in Ag/Fe/Ag, Au/Fe/Ag, Cu/Fe/Ag, Pd/Fe/Ag, and Ni/Fe/Ag structures relative to that of an Au/5.7 ML Fe/Ag reference film. The ratios obtained using method have a total measurement error of a little over 1%. The measurements were carried out to investigate theoretical predictions that Fe atoms located at or near surfaces and interfaces should possess enhanced magnetic moments compared those of Fe atoms in the bulk. All of the bcc Fe(001) films used in our work were approximately 5 monatomic layers (ML) thick and were all grown on bulk fcc Ag(001) substrates. The covering layers of Cu, Pd, and Ag were all 7 ML thick while the covering layers of Ni were 2–3 ML thick and the covering layers of Au were 20 ML thick. All FMR measurements were carried out at a temperature of 77 K. An Ag/5.5 ML Fe/Ag specimen and an Ag/10.9 ML Fe/Ag specimen (the thick Fe specimen) were determined to have a moment of ratio of 1.06 ± 0.01. This compared well to a layer averaged, ground state, moment ratio of 1.05 calculated for the two films using results obtained from published first principles calculations. Ratios of 1.08 ± 0.01 and 1.10 ± 0.01 were obtained for Au/5.7 Fe/Ag and Cu/5.8 Fe/Ag structures, respectively, when compared to the thick Fe specimen. In a similar manner, ratios of 1.11 ± 0.01 and 1.12 ± 0.01, respectively, were determined for Pd/5.6 Fe/Ag and Pd/5.7 Fe/Ag sandwiches. Finally, the layer averaged moment of a 2 Ni/5.7 Fe/Ag structure was in a ratio of 1.15 ± 0.01 with the thick Fe specimen while two 3 Ni/5.7 Fe/Ag sandwiches were in ratios of 1.24 ± 0.01 and 1.20 ± 0.01, respectively. All measured values were in excellent agreement with published first-principles calculations and with the layer averaged moments obtained from polarized neutron reflection (PNR) studies carried out on many of our specimens.

Development of magnetic anisotropies in ultrathin epitaxial films of Fe(001) and Ni(001)

Ultrathin films, bcc Fe(001) on Ag(001), fcc Fe(001) on Cu(001) and Fe/Ni(001) bilayers on Ag, were grown by molecular beam epitaxy. A wide range of surface science tools were employed to establish the quality of epitaxial growth. Ferromagnetic resonance and Brillouin light scattering were used to extract the magnetic properties. Emphasis was placed on the study of magnetic anisotropies. Large uniaxial anisotropies with easy axis perpendicular to the film surface were observed in all ultrathin structures studied. These anisotropies were particularly strong in fcc Fe and bcc Fe films. In sufficiently thin samples the saturation magnetization was oriented perpendicularly to the film surface in the absence of an applied field. It has been demonstrated that in bcc Fe films the uniaxial perpendicular anisotropy originates at the film interfaces. In situ measurements indentified the strength of the uniaxial perpendicular anisotropy constant at the Fe/vacuum, Fe/Ag and Fe/Au interfaces asKus = 0.96, 0.63, and 0.3 ergs/cm2 respectively. The surface anisotropies deduced for [bulk Fe/noble metal] interfaces are in good agreement with the values obtained from ultrathin films. Hence the perpendicular surface ansiotropies originate in the broken symmetry at abrupt interfaces. An observed decrease in the cubic anisotropy in bcc Fe ultrathin films has been explained by the presence of a weak 4th order in-plane surface anisotropy,K1∥S=0.012 ergs/cm2. Fe/Ni bilayers were also investigated. Ni grew in the pure bcc structure for the first 3–6 ML and then transformed to a new structure which exhibited unique magnetic properties. Transformed ultrathin bilayers possessed large inplane 4th order anisotropies far surpassing those observed in bulk Fe and Ni. The large 4th order anisotropies originate in crystallographic defects formed during the Ni lattice transformation.

Ferromagnetic resonance in ultrahigh vacuum of bcc Fe(001) films grown on Ag(001)

Ferromagnetic resonance studies carried out in ultrahigh vacuum at 16.88 GHz on bcc Fe (001) films 5–14.2 monolayers (ML) thick grown on Ag (001) substrates indicate that an ultrathin Fe film 5 ML thick should be magnetized perpendicular to the specimen plane at room temperature. Covering the bare Fe specimens with Ag causes a substantial reduction in the uniaxial surface anisotropy for all Fe film thicknesses and would put the moment of a 5‐ML film back into the plane. For a given Fe film thickness, the maximum obtainable uniaxial surface anisotropy depends on both the amount of oxygen contamination in the film and on the surface roughness.

Engineering magnetic materials on the atomic scale (invited)

Ultrahigh vacuum (UHV) systems and the use of atomic beams for deposition of atoms layer by layer combine to make possible the creation of new materials. The applications to metallic magnetism are gaining increasing attention. The building of sandwiches of magnetic and nonmagnetic layers should lead to increased understanding of the propagation of spin polarization through metals and the effects of finite thickness on the ground state properties and the thermodynamics of magnetic materials. The most important step in this process is in the first layer, i.e., the preparation of the substrate and the determination of the quality of the interface and of the overlayer. The techniques of surface science, e.g., residual gas analysis (RGA), reflection high energy electron diffraction (RHEED), Auger electron spectroscopy (AES), and x‐ray photoemission spectroscopy (XPS) are essential for the characterization of the interface. Illustrations of these include our own work on body‐centered‐cubic Ni deposited epitaxia...

In-Situ Techniques for Studying Epitaxially Grown Layers and Determining their Magnetic Properties

Ultrathin films of bcc Fe (001) on Ag (001) and Fe/Ni (001) bilayers on Ag were grown by molecular beam epitaxy. A wide range of surface science tools (RHEED, REELFS, AES, and XPS) were employed to establish the quality of epitaxial growth. Ferromagnetic resonance and Brillouin light scattering were used to extract the magnetic properties. Emphasis was placed on the study of magnetic anisotropies. Large uniaxial anisotropies with the easy axis perpendicular to the film surface were observed in all ultrathin structures studied. In sufficiently thin samples the saturation magnetization was oriented perpendicular to the film surface in the absence of an applied field. It has been demonstrated that in bcc Fe films the uniaxial perpendicular anisotropy originates at the film interfaces. Fe/Ni bilayers were also investigated. Ni grows in the pure bcc structure for the first 3–6ML and then transforms to a new structure which exhibits unique magnetic properties. Transformed ultrathin bilayers possesses large in-plane 4th order anisotropies far surpassing those observed in bulk Fe and Ni. The large 4th order anisotropies originate in crystallographic defects formed during the Ni lattice transformation.

Probing interfaces of ultra thin films on magnetic substrates using ferromagnetic resonance at 73.55 GHz

Abstract Fe single crystals cut in the (100) orientation are used as substrates for the deposition of ferromagnetic and non-ferromagnetic overlayers. The properties of the overlayer are investigated by measuring the field dependence of the absorption of microwave power, including the ferromagnetic resonance (FMR). Measurements of FMR of (100)Fe with and without an epitaxial body centered cubic (bbc) Ni overlayer are consistent with calculations for a ferromagnetic overlayer with 0.4 μ B per atom of Ni. Epitaxial growth is demonstrated for Ni ( bcc ) on Au and Au on Fe. An epitaxial multilayer has been formed of Au on Ni ( bcc ) on Au on Fe.

Temperature and frequency dependence of microwave magnetic properties of amorphous Fe100−xBx

Ferromagnetic resonance linewidths, ΔH, have been measured at 9.5, 24, 38, and 73 GHz for a series of amorphous iron‐boron ribbons with compositions, Fe100−xBx (x=14 to 22). Transmission measurements at 73 GHz have also been carried out to establish the contribution of the bulk intrinsic damping to the frequency dependence of the FMR linewidth. All measurements were carried out over a temperature range of 20–350 °C. The linewidths of the present specimens could be approximately represented by an expression of the form ΔH=ΔH0+(slope)×f; this observation is in agreement with the results reported on the commercial ribbons. The slope was found to be in agreement with the contribution from the intrinsic Landau–Lifshitz damping as measured by transmission. The Landau–Lifshitz damping was found to increase with decreasing boron concentration.

Ferromagnetic antiresonance transmission through pure iron at 73 GHz

The magnetic damping in pure iron has been studied over the temperature range 140–300 K using a single‐crystal slab l5 μm thick. The results at room temperature are consistent with the accepted value of the damping parameter for iron of G=0.7×108 Hz. The damping increases as the temperature is reduced and reaches a value of approximately 1.4×108 Hz at 140 K.

Epitaxial Growths and Surface Science Techniques Applied to the Case of Ni Overlayers on Single Crystal Fe(001)

The rapidly increasing interest and activity in the study of epitaxially deposited magnetic films on single crystal substrates stem both from the ability to stabilize metastable crystalline structures which do not exist otherwise in nature and from theoretical predictions of enhanced magnetic moments and crystalline anisotropics in low dimensional systems. For example, recent spectacular experimental results1,2 and theoretical calculations3 show that the crystalline anisotropy field in ultrathin Fe films is capable of overcoming the demagnetizing field perpendicular to its surface, making such films an ideal building block for multilayered permanent supermagnets. This is an example of the creation of new magnetic materials by means of atomic engineering. It should be pointed out that such recent advances and future progress in atomic engineering would not be possible without Molecular Beam Epitaxy (MBE) techniques using controlled atomic beams in Ultra High Vacuum (UHV) and using state of the art surface science techniques such as Reflection High Energy Electron Diffraction (RHEED), spin polarized or unpolarized Auger Electron Spectroscopy (AES) and X-Ray Photoelectron Spectroscopy (XPS).